Automatic torque compensating drive system

ABSTRACT

An automatic torque compensating drive system for a hydrostatic transmission, having a torque compensating control mechanism adjusting the swash plate of the hydraulic motor, the swash plate being spring biased to a low-torque, high-speed position and rotatable through a range of positions to a high-torque, low-speed position in response to increasing hydraulic system pressure. When used with a pair of transmissions, a linkage may join the control mechanisms of each transmission to synchronize their torque adjustments.

CROSS-REFERENCE TO RELATED APPLICATION

This application claims the benefit of Provisional Application Ser. No. 61/554,228, filed Nov. 1, 2011, which is incorporated herein by reference in its entirety.

BACKGROUND OF THE INVENTION

This application is related to drive systems for utility vehicles having a prime mover powering the drive system and an auxiliary function, e.g. a pair of hydrostatic transmissions propelling a zero-turn vehicle with a mowing deck. It is desirable that the power directed to the auxiliary function remain constant to produce uniform results, such as a constant blade speed on a mowing deck to provide an even cut. It is further desirable to maintain an overall, constant power output by the prime mover to manage energy consumption. Working against these desired outcomes, increasing demands on the drive system, e.g. terrain of increasing slope, may require the utility vehicle to apportion greater amounts of power from the prime mover to the drive system and away from the auxiliary function, lessening the quality of the auxiliary function. Alternatively, the prime mover may be called upon to provide greater amounts of power to the drive system and the auxiliary function, increasing the utility vehicle's overall energy consumption. A means for adjusting the torque output of the drive system to compensate for increasing demands, while roughly maintaining a constant power draw from the prime mover is desirable.

Torque compensation has been accomplished in the past by manual methods requiring an operator to shift a transmission or motor to a greater mechanical or hydraulic reduction to accomplish an increased torque output as load increases. Such a transition can be abrupt, leading to undesirable ride quality. Automatic torque compensation has been accomplished via hydraulically actuated shift mechanisms piloted by valves reacting to increasing system pressure, and by electrically actuated shift mechanisms in communication with pressure transducers. All of these control methods add complexity and cost not suitable to certain utility vehicles, such as riding mowers.

SUMMARY OF THE INVENTION

The present invention addresses these competing demands by utilizing a drive system having a pair of torque compensating control mechanisms that automatically shift a pair of transmissions through a continuous output range from a low-torque, high-speed mode to a high-torque, low-speed mode as the transmissions experience increasing loads. Thus, under conditions falling within a vehicle's expected duty cycle, power output from the prime mover to the transmissions remains roughly constant as demand changes, allowing the power delivery to an auxiliary function to likewise remain constant.

The present invention may be used with a pair of hydrostatic transmissions, each consisting of a variable speed hydraulic pump subject to manual control, and a variable speed hydraulic motor subject to an automatic torque compensating control mechanism. These control mechanisms take advantage of the natural tendency of a swash plate to seek a full-stroke position as increasing load, manifested as increasing system pressure, rotates the swash plate of the hydraulic motor from a partial-stroke position that corresponds to the low-torque, high-speed mode through a continuous range of positions to the full-stroke position, which corresponds to the high-torque, low-speed mode. This change of position occurs as increasing system pressure in the pistons acts against a thrust bearing located in the swash plate, pivoting the swash plate about its axis of rotation. The swash plate of each hydraulic motor is biased to the partial-stroke position by a spring bias element, allowing the control mechanism to automatically adjust the torque output of the transmission as system pressure, i.e. load, varies. In one embodiment, a pair of stops limits the rotation of the swash plates. Thus, automatic torque compensation is accomplished without needless complexity or cost.

A first objective of the present invention is to accomplish torque compensation in the drive system via a pair of external, spring bias elements acting on the swash plates of the respective hydraulic motors through a camshaft extending out of each transmission's housing. A further objective of the present invention is to link the torque compensating control mechanisms of each transmission to synchronize their torque adjustments. Another objective of the present invention is to accomplish torque compensation via a pair of internal, spring bias elements acting on the swash plates of the respective hydraulic motors.

While the above description has been in connection with a pair of transaxles, it will be understood that the inventions described herein may also be used in other applications, including a single transaxle having a single output axle for other non-vehicle applications or a transaxle for a vehicle such as a lawn tractor and having a pair of output axles extending from opposite sides thereof, such as is shown in commonly owned U.S. Pat. Nos. 7,210,294 and 7,373,871, the terms of which are incorporated herein by reference.

A better understanding of the objects, advantages, features, properties and relationships of the invention will be obtained from the following detailed description and accompanying drawings which set forth illustrative embodiments that are indicative of the various ways in which the principles of the invention may be employed.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is an isometric view of a first embodiment of an automatic torque compensating drive system in accordance with the principles of the invention, where each transmission is biased to a low-torque, high speed mode.

FIG. 2 is an isometric view of the drive system of FIG. 1 with the transmission housings and drive train elements removed for clarity.

FIG. 3 is an isometric view similar to FIG. 2, illustrating the automatic torque compensating drive system self-adjusted to the high-torque, low-speed mode.

FIG. 4 is a top view of the right-side transmission of FIG. 2 illustrating details of the torque compensating control mechanism with certain transmission elements removed for clarity.

FIG. 5 is a top view of the right-side transmission of FIG. 3 illustrating details of the torque compensating control mechanism with certain transmission elements removed for clarity.

FIG. 6 is a reverse, isometric view of certain components of the hydraulic motor shift mechanism of the right-side transmission of FIG. 1, with portions of the transmission housing removed for clarity and the shift mechanism rotated to the partial-stroke position.

FIG. 7 is a view similar to FIG. 6, with the shift mechanism rotated to the full-stroke position.

FIG. 8 is a partially exploded view of components of the hydraulic motor shift mechanism of FIG. 7.

FIG. 9 is a side elevation view of the ramped housing pocket of FIG. 8.

FIG. 10 provides a side elevation view of cooperating components of the hydraulic motor shift mechanism of FIG. 8, namely the camshaft, the cam (from the reverse side depicted therein), and a portion of the shift lever, each component having a clocking feature incorporated into its spline.

FIG. 11 is a top view of a zero-turn mower incorporating the first embodiment of the automatic torque compensating drive system with certain transmission and vehicle elements removed for clarity.

FIG. 12 is side elevation view of an exemplary hydrostatic transmission incorporating an internal torque compensating control mechanism in accordance with the principles of the invention.

FIG. 13 is a partial section view of certain components of the hydrostatic transmission of FIG. 12 along the line 13-13, with certain elements shown whole for clarity and the hydraulic motor in the partial-stroke position.

FIG. 14 is a partial section view similar to FIG. 13, with the hydraulic motor in the full-stroke position.

FIG. 15 is an exploded view of the internal torque compensating control mechanism and the swash plate of the hydraulic motor of FIG. 14.

DETAILED DESCRIPTION OF THE DRAWINGS

The description that follows describes, illustrates and exemplifies one or more embodiments of the present invention in accordance with its principles. This description is not provided to limit the invention to the embodiments described herein, but rather to explain and teach the principles of the invention in order to enable one of ordinary skill in the art to understand these principles and, with that understanding, be able to apply them to practice not only the embodiments described herein, but also other embodiments that may come to mind in accordance with these principles. The scope of the present invention is intended to cover all such embodiments that may fall within the scope of the appended claims, either literally or under the doctrine of equivalents.

It should be noted that in the description and drawings, like or substantially similar elements may be labeled with the same reference numerals. However, sometimes these elements may be labeled with differing numbers or serial numbers in cases where such labeling facilitates a more clear description. For example, the motor swash plates in the two drive system embodiments disclosed herein are serially numbered as 119 and 219, respectively, to ease recognition. Additionally, the drawings set forth herein are not necessarily drawn to scale, and in some instances proportions may have been exaggerated to more clearly depict certain features. As stated above, the present specification is intended to be taken as a whole and interpreted in accordance with the principles of the present invention as taught herein and understood by one of ordinary skill in the art.

The description that follows illustrates representative embodiments of automatic torque compensating drive systems in accordance with the principles of the invention. Each drive system has a unique pair of torque compensating control mechanisms acting upon the hydraulic motors of a pair of hydrostatic transmissions. It should be noted that the term ‘transmission’ is used interchangeably herein to represent both transmissions and transaxles, the latter having a final output axle. The design, layout and function of the exemplary hydrostatic transmissions depicted herein can be substantially similar to that illustrated in commonly-owned U.S. Pat. No. 7,487,636, the terms of which are incorporated by reference herein, and as such, only differences will be described in detail. The primary difference between the transmissions illustrated in U.S. Pat. No. 7,487,636 and those illustrated herein is the presence of an adjustment mechanism associated with the swash plate of the hydraulic motor, whereby the hydraulic motor has a variable displacement as opposed to a fixed displacement.

FIGS. 1-10 illustrate a first embodiment of an automatic torque compensating drive system 10 for a utility vehicle, the drive system 10 having a left-side transmission 101 and a right-side transmission 102, with each transmission 101, 102 having a torque compensating control mechanism 100. The transmissions 101, 102 are left-hand and right-hand versions of substantially similar transmissions, whereby the various control and actuator arms of each transmission move in mirror-image fashion. Each transmission is disposed in a housing 104 consisting of a main housing 104 a and a side housing 104 b, the housing members joined by a plurality of fasteners 105 and sealed in a known manner, such as a liquid sealant or a flexible gasket. The transmissions 101, 102 include a variable displacement hydraulic pump 111 and variable displacement hydraulic motor 112 disposed on a common center section 120. Each transmission 101, 102 may further comprise a planetary reduction (not shown) drivingly engaged to the motor shaft 106 and an output axle 107. Other gear reductions, such as a series of spur gears, are contemplated within the scope of the invention. A wheel hub 108 engaged to the output axle 107 transfers torque from the transmission 101, 102 to a vehicle wheel 109 (as depicted in FIG. 11).

Input shaft 103 may be driven by a prime mover 110, such as an internal combustion engine or electric motor, by means of a belt and pulley system (not shown), and correspondingly drives the cylinder block 114 of hydraulic pump 111. Trunnion mounted swash plate 116, manually operable via control arm 117, controls the displacement of hydraulic pump 111. Hydraulic fluid from hydraulic pump 111 drives hydraulic motor 112 on center section 120 via integral porting 121, 122 (as shown in FIGS. 4 and 5).

Hydraulic motor 112 comprises a cylinder block 115 rotatably disposed on and in fluid communication with center section 120. A plurality of pistons 113 disposed in motor cylinder block 115 is axially displaced by the hydraulic fluid flow from hydraulic pump 111. The pistons 113 continuously engage a thrust bearing 118 seated on a first face of motor swash plate 119. The displacement of hydraulic motor 112 is controlled by the angle of motor swash plate 119, which is pivotable through a range of positions from the partial-stroke position best illustrated in FIGS. 2 and 4 to a full-stroke position illustrated in FIGS. 3 and 5. Motor swash plate 119 has an integral pivot element 119 a formed on the face of motor swash plate 119 opposite the thrust bearing 118. The pivot element 119 a rides in a corresponding channel 130 formed in main housing 104 a. The pivot element 119 a is located off-center to encourage the motor swash plate 119 to move to a full-stroke position in the event the torque compensating control mechanism 100 fails (i.e. ceases to influence the position of motor swash plate 119). In that event, the balance of forces exerted on motor swash plate 119 by the pistons 113 is such that motor swash plate 119 is rotated to the full-stroke position. Accordingly, the hydraulic reduction between hydraulic pump 111 and hydraulic motor 112 is maximized, slowing the rotational speed of output axle 107 and providing a margin of safety.

Each transmission 101, 102 of drive system 10 has a torque compensating control mechanism 100 consisting, in part, of a cam 140 operatively engaged to motor swash plate 119, with cam 140 riding on camshaft 141. Cam 140 is slip fit onto the splines 141 a of camshaft 141 and moves axially along camshaft 141 while rotationally interacting with a set of ramped surfaces 131 a-131 d formed in a receiving or shift pocket 131 (as best shown in FIG. 9). More specifically, a pair of opposing ramped surfaces on cam 140 (as best shown in FIG. 10), i.e. ramped surfaces 140 a and 140 d or ramped surfaces 140 b and 140 c, slidingly engage a corresponding pair of opposing ramped surfaces in shift pocket 131, i.e. ramped surfaces 131 a and 131 d or ramped surfaces 131 b and 131 c, respectively, depending on the direction of rotation of cam 140. A flange 141 b locates camshaft 141 at the base of shift pocket 131, camshaft 141 passing therethrough via bore 133 (as shown in FIG. 8) to the exterior of main housing 104 a. Camshaft 141 is retained in axial alignment in bore 133 by a retaining ring (not shown) installed in retaining ring groove 141 c accessible from the exterior of main housing 104 a when camshaft 141 is installed. A shaft seal (not shown), as known in the art, prevents hydraulic fluid from escaping housing 104 along camshaft 141. FIGS. 4 and 6 depict the position of cam 140 along the length of camshaft 141 at the partial-stroke position corresponding to FIG. 2. FIGS. 5 and 7 depict the position of cam 140 along the length of camshaft 141 at the full-stroke position corresponding to FIG. 3. It should be noted that at full-stroke, a stop surface 119 b of motor swash plate 119 adjacent pivot element 119 a comes to rest against a corresponding stop surface 132 of main housing 104 a. In the partial-stroke position, however, only pivot element 119 a remains in contact with main housing 104 a.

A shift lever 143 engages an end of camshaft 141 external to main housing 104 a, and is secured there by a retaining clip 146 mated to an annular groove 141 d at the external end of camshaft 141. A limiting or lockdown arm 142 having a pair of stops 142 a, 142 b is fastened to main housing 104 a adjacent shift lever 143 by means of a lockdown screw 144 and lockdown washer 145. When appropriately set, the stops 142 a, 142 b correspond to the full-stroke position and the partial-stroke position, respectively, of motor swash plate 119, arresting the movement of shift lever 143 therebetween. As illustrated in FIG. 7, shift lever 143 is positioned vertically in the full-stroke position. This orientation is established by clocking features 150 integrated into the splines of the cam 140, camshaft 141, and shift lever 143 (as illustrated in FIGS. 8 and 10), that align these components to allow cam 140 to recede into shift pocket 131 when shift lever 143 is vertically oriented. The rotational position of lockdown arm 142 at installation is ideally established on a test stand that senses the rotational speed and direction of the output axle 107. Lockdown arm 142 has a pair of slots 142 c, 142 d that permit the use of a single part on both the left-side transmission 101 and right-side transmission 102. Lockdown screw 144 passes through lockdown washer 145 and either slot 142 c or slot 142 d, depending on whether transmission 101 or 102 is being set, to engage a threaded boss (not shown) in main housing 104 a. This setting operation is performed in conjunction with a second test stand operation that establishes a consistent, maximum speed at the output axle 107. This second operation may be accomplished by setting the rotational position of an additional lockdown arm (not shown) that arrests the forward stroke of hydraulic pump 111 by engaging a tab 117 a on control arm 117 to limit its further rotation.

It should be noted that the combination of the shift lever 143, lockdown arm 142, camshaft 141, cam 140, and motor swash plate 119 could be manually operated by a shift linkage (not shown) engaged to the shift lever 143, effectively rendering each transmission 101, 102 a two-speed drive apparatus. Such a structure is known and available in, e.g., the integrated transaxle sold by Hydro-Gear Limited Partnership under the trade name ZT-5400 Powertrain®. In such transaxles, the described shift linkage prevents motor swash plate 119 from self-adjusting with load, free from any operator control as is taught in the present application.

A support rod 160 fastened to a pair of mounting bosses 104 c formed on the main housings 104 a of transmissions 101, 102 provides stiffness to the drive system arrangement. A pair of bias springs 161, each attached to the support rod 160 at a first end, communicates with the respective shift levers 143 of the transmissions 101, 102 at a second end. Though the bias springs 161 are depicted as engaging the support rod 160 via a hook-and-opening type arrangement, other means of engagement known in the art, such as a groove-and-hook type arrangement could be utilized. It should be noted that bias springs 161 could also be anchored to an appropriately located vehicle frame member as opposed to support rod 160. Bias springs 161 act to position the shift levers 143 of each transmission 101, 102, and consequently their respective motor swash plates 119, in the partial-stroke position.

It should be noted that the architecture of cam 140 and shift pocket 131 are such that the partial-stroke position can be achieved by rotating the shift lever 143 clockwise or counterclockwise from the vertical, full-stroke position. As such, lockdown arm 142 may be oriented such that stop 142B serves to establish the vertical, full-stroke position of shift lever 143, and stop 142 a serves to establish the partial-stroke position. In that instance, bias springs 161 would need to be oriented 180° from the direction depicted herein to continue to bias swash plate 119 to the partial-stroke position. Additional flexibility in the positioning of shift lever 143 and its direction of travel is made possible by the clocking features 150 of camshaft 141 and shift lever 143, permitting the shift lever 143 to be mounted in a downward orientation. Such flexibility may help to avoid an interference condition in a given vehicle or machine application.

FIGS. 4 and 5 best illustrate the interaction of camshaft 141, cam 140 and motor swash plate 119 during the transition of hydraulic motor 112 from the partial-stroke position to the full-stroke position as the hydraulic system load increases. FIG. 4 corresponds to the partial-stroke position for hydraulic motor 112, which is the default position for motor swash plate 119. For a given displacement of hydraulic pump 111, this positioning of motor swash plate 119 represents a low(er)-torque, high(er)-speed output as compared to the full-stroke position. The hydraulic reduction between pump 111 and motor 112 in this instance is less than that of the full-stroke position. In this position, the forces applied to motor swash plate 119 by pistons 113 are not sufficient to overcome the bias force applied to motor swash plate 119 by bias spring 161 through shift lever 143, camshaft 141, and cam 140. In a zero-turn mower, for example, this position is suitable for mowing relatively flat terrain or traveling between mowing locations. Allowing greater ground speed is beneficial under these circumstances. In this partial-stroke position, shift lever 143 rests against stop 142 b, and cam 140 is at its maximum internal travel along camshaft 141. A flat face 140 e on cam 140 adjacent motor swash plate 119 operatively engages a curved surface 119 c of swash plate 119 (as best shown in FIGS. 4 and 5), permitting the relative movement of elements as the motor swash plate 119 transitions between positions. FIG. 6 depicts the interaction of cam 140 and shift pocket 131 in this position.

As hydraulic system pressure increases with load, the pistons 113 exert an increasing force against the thrust bearing 118 and motor swash plate 119, ultimately overcoming the bias force of bias spring 161. Curved surface 119 c drives cam 140 against shift pocket 131, wherein a pair of the ramped surfaces 140 a-140 d of cam 140 slidingly engage a pair of the corresponding ramped surfaces 131 a-131 d of shift pocket 131 as previously described, causing the cam 140 to rotate further into shift pocket 131 while moving axially along the splines 141 a of camshaft 141. The rotation of cam 140 causes rotation of camshaft 141 and shift lever 143. Shift lever 143 is moved away from stop 142 b, against the spring force of bias spring 161 to which it is engaged.

As hydraulic system pressure continues to increase with load, the force exerted by pistons 113 against the thrust bearing 118 and motor swash plate 119 ultimately pivots motor swash plate 119 to the full-stroke position, bringing shift lever 143 into engagement with stop 142 a and recessing cam 140 to its greatest extent within shift pocket 131. The positions of the various control elements at the full-stroke position are illustrated in FIGS. 5 and 7. When swash plate 119 is at full-stroke position, bias spring 161 attains its maximum extension, storing energy thereby to return motor swash plate 119 to its partial-stroke position (or to any position in its range of transient positions between the default partial-stroke position and its full-stroke position) as hydraulic system pressure decreases. This full-stroke position represents a maximum hydraulic reduction, which results in high(er) torque and low(er) speed at output axle 107 than derived from the partial-stroke position. In a zero-turn mower, for example, this position may correspond to travel up inclines or through tall grass, where greater torque is required. Any further increase in hydraulic system pressure is preferably relieved by pressure relief valves (not shown) in transmissions 101, 102 in a known manner, preventing damage or reducing wear to internal components over time.

As hydraulic system pressure decreases with a lessening load, the energy stored in bias spring 161 is released, pulling shift lever 143 away from stop 142 a and rotating camshaft 141. Cam 140 is thus rotated and, acting against shift pocket 131, moves axially along camshaft 141 to pivot motor swash plate 119 toward its partial-stroke position.

Accordingly, as hydraulic system pressure varies, so too does the torque output of the transmissions 101, 102 included in drive system 10, simply and automatically adjusting torque in response to load to improve the drivability of any utility vehicle so equipped. To further improve drivability, one may link the responses of the transmissions 101, 102 under load by providing a rigid linkage 170 engaged to the shift levers 143 of each transmission 101, 102 to synchronize their responses to varying loads. As shown installed in the vehicle 180 of FIG. 11, a pivot rod 173 spans the width of vehicle frame 181, rotatably supported therein by bushing mounts 174. A pair of connecting links 172 is fixed to the pivot rod 173 at their first ends to rotate with the pivot rod 173. A connecting rod 171 links a second end of each connecting link 172 with the shift lever 143 of the respective transmission 101, 102. It should be noted that although the bias springs 161 are depicted as engaging the connecting rods 171 of each transmission 101, 102, the bias springs 161 could also directly engage an opening (not shown) on the respective shift levers or control arms 143. It should also be understood that a more basic linkage could be used where clearance permits. For example, a rigidly fixed tie rod could be extended between the two control arms 143, and one end of each of the springs 161 could be affixed to this tie rod instead of to the separate control arms 143, to accomplish this linkage of the two transmissions.

FIGS. 1 and 2 illustrate the position of the linkage 170 when the hydraulic motors 112 of transmissions 101, 102 are in partial-stroke. FIG. 3 illustrates the rotated position of the linkage 170 when the hydraulic motors 112 of transmissions 101, 102 are in full-stroke, or maximum hydraulic reduction. It should be noted that each transmission may experience different loads, e.g. when a zero-turn vehicle 180 such as that depicted in FIG. 11 laterally traverses a hillside, the loading on the downhill transmission is greater than that of the uphill transmission. As a result, the downhill transmission requires additional torque to keep the vehicle 180 tracking in a straight line across the face of the hill. In such a situation, the downhill transmission may seek to rotate its motor swash plate 119 toward the full-stroke position, while the uphill transmission may not. Absent the linkage 170, this event could cause the vehicle to turn unexpectedly, or to a greater degree than expected, as the change in relative output speeds of the two transmissions 101, 102 produces a greater turn than gravity alone. Linkage 170 synchronizes the strokes of the two hydraulic motors 112 in response to one or both of the transmissions making an adjustment in torque. Consequently, the operator of vehicle 180 may simply concentrate on the control inputs to each of the hydraulic pumps 111 via control linkages (not shown) to the respective control arms 117 of each transmission 101, 102.

Distinguishing portions of a second embodiment of a drive system are illustrated in FIGS. 12-15, where a representative transmission 201 is equipped with a fully internal, torque compensating control mechanism 200. In this second embodiment, camshaft 241 does not extend outside main housing 204 a, in contrast to the previously-described torque compensating control mechanism 100 of drive system 10, where the camshafts 141 of transmissions 101, 102 extend outside main housing 104 a. Accordingly, a shift lever, external bias spring, and rigid linkage are not present. Though a single, left-side transmission 201 is depicted, and may be used in cooperation with utility equipment (not shown) requiring a single output, it is also to be understood that a corresponding right-side transmission having substantially identical features may complete a drive system similar to the drive system 10 depicted in FIG. 1.

The internal, torque compensating control mechanism 200 consists of a camshaft 241 having a radial flange 241 b approximately midway along its longitudinal axis, dividing the camshaft into two portions; a first portion 241 e that resides in a bore 233 connecting a receiving or shift pocket 231 to the exterior of main housing 204 a, and a second portion 241 a that receives a cam 240 and a wave spring 261 disposed between the radial flange 241 b and cam 240. Unlike the splined interface of camshaft 141 and cam 140, camshaft 241 and cam 240 have smooth mating surfaces 241 a, 240 f, it being unnecessary to rotate camshaft 241. A retaining ring 247, or similar element, retains cam 240 and wave spring 261 on camshaft 241, engaged there to a retaining ring groove 241 d. This cam assembly 249 is preferably assembled prior to insertion through shift pocket 231 and bore 233. Radial flange 241 b axially positions the cam assembly 249 within shift pocket 231 and bore 233. A second retaining ring 248 interfaces with a corresponding groove 241 c on the first portion 241 e of camshaft 241 protruding from bore 233, retaining it therein. A sealing plug 251, such as a rubber coated metal seal, is press fit into the stepped counter-bore 234 leading to bore 233, sealing main housing 204 a.

FIGS. 13 and 14 depict the partial-stroke and full-stroke positions, respectively, of motor swash plate 219. The interaction of motor swash plate 219, cam 240, and shift pocket 231 in response to changes in hydraulic system pressure, i.e. changing loads, is similar to that previously described for the torque compensating control mechanisms 100 of drive system 10. Wave spring 261 provides the bias force previously supplied by external bias spring 161, exerting force against the radial flange 241 b of camshaft 241 and against cam 240. At a hydraulic system pressure insufficient to overcome the spring force of wave spring 261, motor swash plate 219 resides in the partial-stroke position, its curved surface 219 c operatively engaged to a flat face 240 e of cam 240. In the partial-stroke position, only pivot element 219 a of swash plate 219 is in contact with main housing 204 a.

As hydraulic system pressure increases to a point where the spring force of wave spring 261 is exceeded, the stroke angle of swash plate 219 begins to increase, causing cam 240 to be driven into shift pocket 231 as wave spring 261 is compressed. As previously described for the interaction of cam 140 and shift pocket 131, a pair of opposed ramped surfaces from the set of ramped surfaces 240 a-240 d on cam 240 slidingly engage a pair of the corresponding ramped surfaces (not shown) in shift pocket 231 to rotate cam 240 as it is driven deeper into shift pocket 231 with increasing hydraulic system pressure. As hydraulic system pressure continues to build in response to increased load, the full-stroke position of FIG. 14 is achieved with cam 240 receding a maximum distance into shift pocket 231, wave spring 261 reaching maximum compression, and stop surface 219 b of swash plate 219 coming to rest against main housing stop surface 232. This position represents the maximum hydraulic reduction between a hydraulic pump (not shown) in transmission 201 and hydraulic motor 212, and therefore, a high(er) torque, low(er) speed state for transmission 201. A depression 219 d adjacent curved surface 219 c of swash plate 219 provides clearance for the end of camshaft 241, preventing contact between the two elements at the full-stroke position.

When hydraulic system pressure begins to decrease with decreasing load, the energy stored in the compressed wave spring 261 is released, driving cam 240 partially out of shift pocket 231 and, ultimately, returning swash plate 219 to the partial-stroke position. This position represents a low(er) torque, high(er) speed state for transmission 201. It should be noted that for internal, torque compensating control mechanism 200, without any need to rotate camshaft 241, shift pocket 231 and cam 240 may be simplified to a piston and straight bore design. (Even with the cam design, once cam 240 recedes its maximum distance into shift pocket 231, it may thereafter just slide axially along the smooth exterior of camshaft 241, sans rotation, as torque is regulated.) Furthermore, bore 233 need not open to the external side of main housing 204 a, eliminating a potential leak path for hydraulic fluid and the need for sealing plug 251. As currently illustrated, however, a single design for main housings 104 a, 204 a beneficially accommodates both torque compensating control mechanisms 100, 200.

As previously mentioned, while torque compensating control mechanism 200 has been illustrated as applied to a transmission 201 having a single output axle, as part of a two transmission drive system, the device could also be employed in a drive system utilizing a single transmission driving a pair of output axles, such as that shown in U.S. Pat. Nos. 7,373,871 and 7,210,294.

While specific embodiments of the invention have been described in detail, it will be appreciated by those skilled in the art that various modifications and alternatives to those details could be developed in light of the overall teaching of the disclosure. Accordingly, the particular arrangements disclosed are meant to be illustrative only and not limiting as to the scope of the invention. 

What is claimed is:
 1. A drive system, comprising: a first transmission disposed in a first housing and producing a first output; and a first torque control mechanism engaged to the first transmission to adjust the torque level of the first output to produce a continuous range of torque from a low-torque value to a high-torque value, the first torque control mechanism comprising a first biasing member to bias the first output to the low-torque value; a second transmission, substantially similar to the first transmission, disposed in a second housing and capable of producing a second output substantially similar to the first output; a second torque control mechanism engaged to the second transmission capable of adjusting the torque level of the second output to produce a continuous range of torque from the low-torque value to the high-torque value, the second torque control mechanism comprising a second biasing member to bias the second output to the low-torque value; wherein the low-torque value corresponds to a high rotational speed at the first and second outputs, and the high-torque value corresponds to a low rotational speed at the first and second outputs.
 2. The drive system of claim 1, wherein the first torque control mechanism automatically adjusts the output of the first transmission in response to load, and the second torque control mechanism automatically adjusts the output of the second transmission in response to load.
 3. The drive system of claim 2, wherein the first biasing member is located external to the first housing and the second biasing member is located external to the second housing.
 4. The drive system of claim 3, wherein the first biasing member and the second biasing member comprise a first spring and a second spring, respectively.
 5. The drive system of claim 4, further comprising an operative linkage between the first torque control mechanism and the second torque control mechanism, wherein the operative linkage causes the first output and the second output to be adjusted to substantially similar torque values and rotational speeds by their respective torque control mechanisms.
 6. The drive system of claim 5, further comprising a support engaged to the first housing and the second housing, wherein a first end of the first spring and a first end of the second spring are attached to the support, and a second end of the first spring and a second end of the second spring are attached to the operative linkage.
 7. The drive system of claim 1, wherein the first torque control mechanism adjusts the output of the first transmission toward the high-torque value as the load experienced by the first transmission increases, and the second torque control mechanism adjusts the output of the second transmission toward the high-torque value as the load experienced by the second transmission increases.
 8. The drive system of claim 7, wherein the first torque control mechanism adjusts the output of the first transmission toward the low-torque value as the load experienced by the first transmission decreases, and the second torque control mechanism adjusts the output of the second transmission toward the low-torque value as the load experienced by the second transmission decreases.
 9. The drive system of claim 8, wherein the first transmission comprises a first axial piston hydraulic motor and the second transmission comprises a second axial piston hydraulic motor, wherein the first axial piston hydraulic motor and the second axial piston hydraulic motor each comprise a swash plate pivotable between a first position corresponding to the low-torque value and a second position corresponding to the high-torque value, and each swash plate comprises an integrally formed pivot surface that rides in a corresponding channel formed in the respective first and second housings.
 10. The drive system of claim 9, further comprising a pair of cam members, each operatively engaged to one of the swash plates and its respective first or second biasing member, wherein each cam member acts upon the respective swash plate under motive force from the respective first or second biasing member to drive the respective swash plate to the first position, and wherein each cam member is forced into a receiving pocket in the respective first or second housing by the respective swash plate as the respective swash plate moves to the second position.
 11. The drive system of claim 10, wherein each of the pair of cam members is fixed to a shaft extending outside of the respective first and second housings to engage the respective first and second biasing members, the engagement made via first and second control arms, respectively.
 12. The drive system of claim 11, wherein the first transmission and the second transmission each further comprises a limiting arm having a first stop to limit travel of the respective control arm corresponding to the first position of the respective swash plate and a second stop to limit travel of the respective control arm corresponding to the second position of the respective swash plate.
 13. The drive system of claim 12, wherein the first biasing member comprises a first spring and the second biasing member comprises a second spring.
 14. The drive system of claim 1, wherein the first biasing member comprises a first spring disposed in a first receiving pocket formed in the first housing and the second biasing member comprises a second spring disposed in a second receiving pocket of the second housing.
 15. A vehicle comprising: a frame; a prime mover mounted to the frame; a first transmission driven by the prime mover and mounted to the frame, wherein the first transmission comprises a first torque control mechanism and a first output axle, wherein the first torque control mechanism adjusts the first transmission to produce a continuous range of torque at the first output axle, the torque ranging from a low-torque value to a high-torque value; a second transmission, substantially similar to the first transmission, driven by the prime mover and mounted to the frame, wherein the second transmission comprises a second torque control mechanism and a second output axle, wherein the second torque control mechanism adjusts the second transmission to produce a continuous range of torque at the second output axle, the torque ranging from the low-torque value to the high-torque value; a first wheel driven by the first output axle and a second wheel driven by the second output axle; a first biasing member engaged to the first torque control mechanism, wherein the output of the first transmission is biased to the low-torque value; a second biasing member engaged to the second torque control mechanism, wherein the output of the second transmission is biased to the low-torque value; wherein the low-torque value corresponds to a high rotational speed at the first and second output axles, and the high-torque value corresponds to a low rotational speed at the first and second output axles.
 16. The vehicle of claim 15, wherein the first biasing member is disposed external to the first transmission and the second biasing member is disposed external to the second transmission.
 17. The vehicle of claim 16, further comprising an operative linkage between the first torque control mechanism and the second torque control mechanism, wherein the operative linkage causes the outputs of the first and second transmissions to be adjusted to substantially similar torque values and rotational speeds by their respective torque control mechanisms.
 18. The vehicle of claim 15, wherein the first transmission and the second transmission each further comprise a hydrostatic transmission having a hydraulic pump and hydraulic motor in fluid communication, and the first torque control mechanism acts on the hydraulic motor of the first transmission to adjust the torque at the first output axle, and the second torque control mechanism acts on the hydraulic motor of the second transmission to adjust the torque at the second output axle.
 19. The vehicle of claim 15, wherein the first biasing member is located internal to the first transmission and the second biasing member is located internal to the second transmission.
 20. The vehicle of claim 19, wherein the first biasing member comprises a first spring disposed on a first cam shaft located in a first receiving pocket formed in the a first housing for the first transmission, and the second biasing member comprises a second spring disposed on a second cam shaft located in a second receiving pocket formed in a second housing for the second transmission.
 21. A transmission system comprising: a housing; a motor disposed in the housing and having a torque output; an adjustable motor swash plate operatively engaged to the motor to control the torque output of the motor; a pump disposed in the housing and having a hydraulic output operatively connected to the motor; an adjustable pump swash plate operatively engaged to the pump to control the hydraulic output of the pump; and a torque control mechanism engaged to the motor swash plate to adjust the torque output of the motor to produce a continuous range of torque from a low-torque value to a high-torque value, the torque control mechanism comprising a biasing member to bias the torque output to the low-torque value.
 22. The transmission system of claim 21, wherein the torque control mechanism comprises: a cam shaft extending into the housing; a cam disposed on the cam shaft and having a cam surface engaged to the motor swash plate; and a spring disposed on the cam shaft between the cam and the housing to bias the swash plate to a position corresponding to the low-torque value.
 23. A drive system comprising a transmission disposed in a housing and producing an output, and a torque control mechanism engaged to the transmission to adjust the torque level of the output to produce a continuous range of torque from a low-torque value to a high-torque value, the torque control mechanism comprising a biasing member to bias the output of the transmission to the low-torque value.
 24. The drive system of claim 23, wherein the biasing member comprises a spring disposed in a receiving pocket formed in the housing.
 25. The drive system of claim 24, wherein the torque control mechanism further comprises a cam shaft extending into the housing, a cam disposed on the cam shaft and having a cam surface engaged to a motor swash plate, and the spring is disposed on the cam shaft between the cam and the housing to bias the motor swash plate to a position corresponding to the low-torque value. 